1. Field
The present invention relates to communications technology, and more particularly to a beam forming method, apparatus and system in wireless communications technology.
2. Description of the Related Art
In communications technology, at high carrier frequencies, the path loss is significantly larger than with small carrier frequencies. Thus, e.g. at a frequency of 2.5 GHz, like for a wireless metropolitan area network technology WiMAX (Worldwide Interoperability for Microwave Access), the path loss is significantly larger than when operating with e.g. GSM/EDGE (Global System for Mobile Communications/Enhanced Data Rates for Global Evolution) system or UMTS (Universal Mobile Telecommunications System) system at 900 MHz. This leads to the fact that it is difficult to obtain large cell ranges and adequate coverage with port-based network access standard 802.16 or standard 802.16e communications without large radio transmitter TX powers and large high-gain antennas.
Some solutions have been proposed to 2G wireless communication systems, which try to apply beamforming techniques to solve the problem, i.e. how a standard 2-antenna base station could take advantage of a multi-antenna array. However, the problem with such 2G systems is that the composite downlink signal cannot be decomposed to user-specific data and control signals at the external beamforming unit. Moreover, the problem of transmitting a reference signal (i.e. a pilot/training signal) to the entire sector coverage jointly with beamforming cannot be solved by these systems.
A state-of-the-art remote radio head (RRH) unit can be described to be a part of a distributed base station system in which all radio-related functions are contained in a unit mounted outside a traditional base station transceiver system BTS but linked to a main transceiver unit that contains the control and baseband signal processing functions.
Most cellular systems apply sectorized base stations, which have up to 40 W (watt) of TX power per sector and which utilize large, up to 24 dBi (decibel isotropic) sector-antennas. However, the most advanced commercial 2G/3G base stations employ fixed beam approach or other antenna signal combining methods, which require beam- or antenna-specific signal processing in base band. Thus e.g. a 8-beam antenna array requires 8 cables and 8 radio frequency RF chains between the antenna unit, i.e. the RRH unit and the base station, i.e. the main transceiver unit. However, all these proposed solutions fail to disclose, how to design a simple base station and a mobile terminal with limited signal processing requirements, which is power-efficient but still has good coverage and link budget.